1. Field of the Invention
The present invention relates generally to radar processing systems, and particularly to Ground Moving Target Indicator (GMTI) radar with adaptive clutter suppression.
2. Technical Background
The term radar is an acronym that stands for “radio detection and ranging.” A radar system transmits radio frequency (RF) signals in a predetermined direction (i.e., a bearing) with the intention of contacting or illuminating moving objects (“contacts”). When the transmitted radar signal illuminates a contact, a return signal is reflected back toward the radar receiver. The return signal is detected if it is stronger than any noise signals that may be present in the receiver. A contact's bearing corresponds to the direction of the transmitted radar signal. Because the signal travels at the speed of light, the distance, or “range”, is determined by measuring the time between signal transmission and the reception of the return signal. Radar has proved to be a very useful tool that can detect contacts such as spacecraft, aircraft, vehicles, etc., within a predetermined region or search volume and provide the radar receiver with the contacts bearing, range, velocity, etc. This information provides military commanders, security personnel, or police with the intelligence they needed to properly assess their situational awareness. Moreover, radar systems are now being used in many different applications including civilian air traffic control, search and reconnaissance, weather forecasting and tracking, and automotive traffic control, to name a few. Another radar application that has been garnering a great deal of interest relates to ground moving target indication (GMTI).
GMTI is an important application for “look-down” (i.e., airborne and space based systems) radar systems. Because resources are limited, military commanders must use their assets smartly and efficiently. To do this, they require reliable intelligence in order to develop accurate “situation awareness” (SA). SA is about knowing where the enemy is, how big it is, where it is going and how fast it is getting there.
One of the drawbacks with GMTI radar relates to its ability (or inability) to distinguish slow-moving targets from background clutter. Clutter refers to the radar return signals that are reflected by terrain, buildings, trees and other such objects that are not of interest to the decision makers. GMTI radars use the Doppler Effect to distinguish moving contacts from stationary ones. (When a contact approaches the radar receiver, its velocity component parallel to the line of sight of the radar imparts a positive frequency shift if moving towards the radar, and a negative frequency shift if moving away from the radar. This frequency shift is referred to as Doppler and the relevant velocity component is the Doppler velocity. The use of Doppler radar provides a widely used means for distinguishing a target contact from stationary background (clutter). The Doppler frequency is calculated by calculating the ratio of twice the relative velocity over the wavelength of the radar signal (i.e., FD=(2)(VR)/λ). (This expression assumes a monostatic radar wherein the transmitter and receiver are collocated. In bistatic radar, the expression is modified to account for the differing velocity vector orientation with respect to the transmitter and receiver.) When a radar platform is moving (e.g., it is mounted on an aircraft), however, clutter returns at different angles will appear to move at different velocities and thus impart a spread of Doppler frequencies that can mask a moving target. By filtering both in angle and in Doppler, the radar processor can distinguish between clutter and target unless the target is moving too slowly. In this case, the competing clutter will arise from nearly the same angle as that of the target. This gives rise to the notion of “minimum detectable velocity (MDV).” Briefly stated, if the target is below the radar's MDV it will not be detected; on the other hand, if a contact's Doppler velocity is above the radar's MDV, the GMTI radar can detect the contact.
A limiting factor of GMTI radar arises from the fact that the MDV is primarily limited by the electrical size of the radar antenna aperture; sharper angle filtering requires a larger antenna aperture. The MDV is inversely related to the electrical size of the radar antenna aperture. Thus, the MDV is reduced by increasing the electrical size of the radar antenna aperture. However, since the GMTI radar (and its antenna) is part of the aircraft's payload, the size of the antenna aperture is strictly limited by the size of the aircraft platform itself. What is needed therefore is a way to increase the electrical size of the radar antenna aperture without the physical constraints outlined above.
In one approach that was considered, additional antenna elements were mounted on stationary, distributed platforms. The antennas were widely separated spatially to increase the electrical size of the overall radar antenna aperture. Since the radar processor knew the precise position of each platform's antenna phase center, it also knew a priori what the phase offsets were between antennas. In other words, because the platforms were stationary, the computational burden placed on the processor was significantly reduced, making the system feasible. One obvious drawback to this approach relates to the fact that the system is immobile during usage.
Thus, a major drawback to mobile multiplatform GMTI radar relates to the fact that the relative positions of the platform antenna phase centers must be tracked to a small fraction of a wavelength. Conventional proposals to solve this ambiguity problem, and other problems associated with mobile distributed array multiplatform radar, make the assumption that the array phase centers can be precision tracked to fractions of a wavelength by some means, or that the array can be cohered by focusing on strong scatterers, transponders, and so forth. Such tracking accuracies are very difficult, if not impossible, to achieve with moving platforms at radar frequencies.
What is needed, therefore, is a GMTI radar that can detect ground moving targets with very small minimum detectable velocities by combining signals in a clutter suppressing space-time (angle-Doppler) adaptive processing (STAP) filter without requiring that the relative positions of the antenna phase centers be accurately tracked. As those skilled in the art will appreciate, STAP filters adaptively (automatically) combine temporal and spatial data to produce a null at angles corresponding to clutter and other interference, while simultaneously producing gain at the angles and Doppler frequency of the targets.
FIG. 1A shows the range-Doppler response map for a conventional multichannel GMTI radar system that employs STAP. The target is clearly detected at a range cell of about 370, and exhibits a Doppler frequency that is close to 15 Hz. In this case, the low Doppler frequency indicates that the target has a relatively low MDV. FIG. 1B shows the radar performance in the presence of noise and without STAP. This Figure illustrates the impact that clutter and other types of interference can have on the radar detection process, namely, that the radar cannot detect the target without such adaptive processing. These drawing Figures highlight the need for a technique that can successfully overcome the phase center ambiguity problem and implement mobile multi-platform GMTI radars using adaptive processing such as STAP.
What is needed is a multiplatform GMTI radar (thus large aperture) with adaptive clutter suppression. What is further needed is a simple means of handling the phase center ambiguity of mobile multiplatform radar.